Thawing permafrost is not the only grave problem with Earth’s cryosphere. Ice sheets, especially the massive Greenland and West Antarctic Ice Sheet (WAIS), have discrete temperature thresholds at which near-total loss of ice becomes unstoppable. These two highly vulnerable sheets are now both losing mass much faster than the authoritative Intergovernmental Panel on Climate Change had predicted just a few years ago.

Tipping Points Near for Polar Ice Sheets

A recent report called Thresholds and Closing Windows: Risks of Irreversible Cryosphere Climate Change” by the International Cryosphere Climate Initiative (ICCI), reveals that some portions of these ice sheets may have already begun irreversible melting at today’s elevated global temperature of about 1°C above the pre-industrial average. Things would obviously be worse at the 1.5 ° to 2° aspirational targets set by the December Paris climate accords. And they’d be even worse at the 2.7 ° to 3.5° increase that its current pledges would produce if completely fulfilled—something that a voluntary agreement cannot guarantee.

Research cited by ICCI scientists suggests that somewhere between 1° to 4° above the pre-industrial level, the irreversible melting of Greenland will begin, with 1.6° as the likeliest tipping point.

Greenland and the West Antarctic Ice Sheet together contain enough ice to raise sea level about 50 feet. Even a 30-foot rise from these sources—consistent with what the geological record shows for previous interglacial periods—would be enough to flood areas of the U.S. where a quarter of the population resides, along with trillions of dollars of infrastructure and real estate.

A 30-foot rise would likely take a very long period time—perhaps thousands of years for its completion—but it could be irrevocable just within the next few decades because of irreversibilities in ice sheet dynamics.

Once warming seawater melts the submerged toe of a glacier that was solidly grounded on bedrock or wedged behind a seafloor ridge, it is only a matter of time before the glacier behind the ice dam flows into the sea. Likewise, according to Max Holmes, senior climate scientist at Woods Hole Research Center, when the surface of the Greenland ice cap shrinks enough that its surface is at lower elevation where air temperatures are warmer, a positive melting-and-warming feedback cycle takes over, and eventually nearly all of the Greenland ice sheet may melt away.

Global warming is amplified at high latitudes. Thus, whereas the world’s average temperature gain is now barely 1°C above the pre-industrial level, the Arctic and parts of the Antarctic have on the average already warmed two to three degrees. Moreover, because the warming is not uniform, parts of the Arctic have warmed by more than 8° and some areas by nearly 12°. Thus, it shouldn’t be surprising that half the surface area of Greenland was already melting in the summer of 2015, and that Greenland now loses about 200 billion tons of ice to the ocean a year.

A new study recently published online in Nature by David Pollard, a paleoclimatologist at Pennsylvania State University, and Rob DeConto, a University of Massachusetts geoscientist, agrees that global emissions in just the next few decades could cause irreversible melting in Antarctica that would commit the world to a sea level rise of more than a meter by 2100 and more than 15 meters by 2500.

Arctic Summer Sea Ice Could Vanish

Summer sea ice in the Arctic cools the Earth because it reflects solar heat that otherwise is absorbed by darker less reflective ocean water, warming the planet. But half of the end-of-summer summer sea ice found in the Arctic in 1950 is already gone. This loss has helped drive Arctic temperatures up by multiples of the world’s average temperature rise, accelerating permafrost thawing and Greenland melting.

At the projected 2.7° to 3.5° global temperature increase projected as a result of the emission-reduction pledges made last December at the global climate accord in Paris, all summer sea ice will disappear once temperatures approach 3°, acondition that has never occurred in modern human existence, according to ICCI.

The complete loss of summer sea ice will have significant impacts on the world’s weather, ecosystems, and economy. Greater instability in the jet stream, less stable polar fronts, and more extreme weather in mid-latitudes are expected. Ice-dependent marine food chains and ecosystems may collapse, and traditional Arctic cultures will be devastated.

Polar Ocean Ecosystems Could Collapse

Ocean acidity—stable for millions of years before the industrial revolution—has already increased by 30 percent since then, dropping by 0.1 pH units to pH 8.1. As is well known, the increased acidity is already harming sensitive organisms that require carbonate from seawater to build their shells or exoskeletons, including reefs.

Because cold water takes up carbon dioxide more readily than warmer water and because polar oceans are also freshened by melting ice, they are more susceptible to acidification and are increasing in acidity much faster than the rest of the ocean. These oceans hold some of the world’s richest fisheries.

According to estimates by a number of national academies of science, allowing the Earth’s atmospheric carbon dioxide concentration to rise from its current 407 ppm (as of April 2016) to 450 ppm would be accompanied an overall drop in ocean pH of 0.2 units, causing widespread disruption to marine ecosystems and food webs. The most recent data available show the annual concentration is rising at 3 ppm.

Climate experts who have modeled the Paris pledges have concluded that these allowed emissions would result in an ocean pH of 7.8-7.95. While the atmosphere can recover from an excess of carbon dioxide in a 1,000 years or less, once the oceans are acidified, they will require tens to hundreds of thousands of years to recover, through buffering from the gradual weathering of rocks.

All this is just further evidence that the world needs to go back to the climate negotiating table—and in a big hurry—to make far deeper mandatory cuts in emissions than those that nations pledged in Paris last year.

The views expressed are those of the author(s) and are not necessarily those of Scientific American.

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